Coal-fired combustion systems such as those used in large utility applications require finely-ground coal particles or "fines" for efficient operation. In general, it is desirable to use only very-finely pulverized coal in such systems in order to keep NOX emissions and oversized loss-on-ignition (LOI) unburned coal particles from contaminating the marketable ash byproduct of the combustion chamber. It is accordingly important to maintain close control over the fineness of the pulverized coal fed into the combustion system.
Bowl mill-type pulverizers, such as the type disclosed in U.S. Pat. No. 4,687,145, are commonly used to grind the coal and classify the resulting fines. A vertical feedpipe drops raw coal from several feet above the pulverizer to the center of the pulverizer for grinding. An annular and upwardly-directed flow of air through a ring-shaped "throat" blows the ground coal particles up and around the pulverizer to a classifier system and combustion chute feeding the combustion chamber. The classifier system removes oversized particles of coal from the flow of air and coal fines, returning them to the pulverizer for regrinding.
A known system for classifying these upwardly traveling fines consists of an inverted classifier cone mounted above the pulverizer and concentric with the feedpipe that delivers raw coal to the center of the pulverizer. The lower, smaller outlet end of the classifier cone essentially surrounds the outlet end of the feedpipe, while the larger, upper inlet or mouth of the cone surrounds the combustion delivery chute.
A stationary ring of classifier vanes is mounted at the mouth of the cone to receive the annular, upward flow of pulverized coal/air from the pulverizer and redirect it into the classifier cone in a centrifugal flow. As the coal fines and air swirl around in the classifier cone, the heavier particles gravitate to the sides and settle out at the bottom of the cone, while the lighter, more finely ground fines are swirled up and into the entrance of the combustion delivery chute.
As the heavier particles of coal collect at the bottom of the classifier cone, they are typically contained by a flapper valve assembly at the bottom of the cone, comprising a series of vertically hanging plates blocking the openings of one or more outlet chutes. The plates are relatively heavy, and are forced open only intermittently by the weight of the accumulated coal at the bottom of the classifier cone. These fine "rejects" then fall into the bowl mill pulverizer along with incoming raw coal from the feedpipe for regrinding.
There are a number of disadvantages inherent in prior art systems such as those described above.
The prior art positioning of the feedpipe and classifier cone outlets well above the pulverizer often results in fine rejects being blown back up through or around the classifier cone when the flapper assembly opens for a discharge. This is primarily due to the position of the outlets relative to the annular flow of coal fines/air from the pulverizer throat.
Moreover, the flapper assembly and other prior art intermittent cone discharge systems such as "hula skirt" assemblies (circular arrangements of overlapping metal leaves) can become stuck in an open position, adding to the problem of fine reject backflow into the combustion delivery chute and further defeating the function of the classifier cone.
In U.S. patent application Ser. No. 08/135,726 filed Oct. 13, 1994, I disclosed an improvement upon the prior art coal pulverizer systems described above wherein the various air and coal flow paths throughout the pulverizer are optimized to prevent them from interfering with one another. In one form of that invention the feedpipe and classifier cone are extended to eliminate the adverse effects of the annular fine-lifting air flow from the pulverizer throat on the function of the classifier cone, to improve the intrinsic functioning of the classifier cone, and to eliminate the need for complex and unreliable intermittent discharge structure in the classifier cone.
This is generally achieved by extending the feedpipe and classifier cone such that the drop-points for raw coal from the feedpipe and for reject fines from the cone are within, rather than above, the pulverizer. These extensions significantly reduce the tendency of the annular, fine-lifting flow around the outside of the pulverizer to deviate and work back up against the flow of raw coal and fine rejects into the pulverizer.
The reject fines spiraling down the cone around the feedpipe are further drawn into the pulverizer by an improved pressure flow effect from the extended feedpipe in a manner which prevents diversion of the fines back up into or around the classifier cone.
In a particular embodiment of my previous invention, the flapper valve or other intermittent discharge structure is removed from the classifier cone outlet, and replaced with a continuous flow feedpipe extension extending well below the original classifier cone outlet to a point proximate the grinding surface of the pulverizer. The classifier cone is extended in similar fashion with an extension concentric with the feedpipe and extending into the pulverizer to a point proximate the feedpipe extension outlet. In a preferred form the classifier cone extension extends into the pulverizer slightly farther than the feedpipe extension, with its outlet slightly below that of the feedpipe extension, such that the raw coal flow through the feedpipe creates a desirable pressure flow effect drawing the reject fines from the cone into the pulverizer.
The continuous-flow feedpipe and classifier cone extensions, when properly adjusted relative to the pulverizer and its annular fine-lifting airflow, provide a steady flow equilibrium not attainable with the intermittent discharge structure which they replace.
Occasionally, however, the dimensions and geometry of the feedpipe and the classifier cone are such relative to one another that the feedpipe extension interferes with the flow of coal fines through the classifier cone extension. For example, the feedpipe may be oversize in diameter to accommodate a required feed rate, with the result that the feedpipe extension fits too closely within the classifier cone extension for proper flow between the feedpipe extension and the cone extension.